The long term objective of this research is to identify molecular targets for neuroprotective therapies in multiple sclerosis (MS). It is now recognized that axon damage occurs commonly in MS and is an important cause of permanent disability. Developing neuroprotective therapies that halt axonal degeneration is a major therapeutic goal of MS research. Although the precise mechanisms leading to axonal degeneration are poorly understood, they most likely stem from a cascade of ionic imbalances initiated by mitochondrial dysfunction and concomitant deficits in cellular energy supply, ultimately resulting in mitochondrial and axonal Ca2+ overload. Using the murine model of MS, experimental autoimmune encephalomyelitis (EAE), we have demonstrated that mouse mutants missing cyclophilin D (CyPD-KO), a key regulator of the mitochondrial permeability transition (PT) pore and the major pathway for Ca2+ release from mitochondria, have dramatically reduced axonal damage compared with wild type (WT) mice despite the presence of inflammation within the central nervous system (CNS). CyPD-KO mice develop acute EAE similar to WT mice but unlike the WT mice, recover clinically and show up to an 80% reduction in axonal damage. Importantly, mitochondria from CyPD-KO mice are resistant to Ca2+-mediated PT Pore activation and primary cortical neurons from CyPD-KO mice resist injury induced by oxygen and nitrogen free radicals, mediators of injury in EAE and MS. These results suggest a critical role for the PT Pore in determining the fate of axons in EAE and MS. The guiding hypothesis of this proposal is that modulation of the PT Pore by inactivation of CyPD will enhance the ability of axonal mitochondria to sequester Ca2+ in response to pathologic increases in Ca2+, thereby delaying activation of the PT Pore. In turn, inhibition of PT Pore activation will abrogate ATP depletion, axoplasmic Ca2+ overload, and the initiation of a molecular cascade that leads to axonal destruction. We propose the following specific aims to further test the role of the mitochondrial PT Pore and its modulation by CyPD inactivation in the development of axonal injury in EAE. In Aim 1, we will use CyPDloxP/neuronal Cre mice to determine whether inactivation of CyPD in neurons and their axons and not in other CNS cell types results in axonal protection in EAE. In Aim 2, we will use primary cortical neuronal cultures from WT and CyPD-KO mice to determine whether toxic inflammatory mediators generated during EAE 1) change dendritic stability and neuronal viability in WT neurons and 2) increase mitochondrial Ca2+ levels and activate the PT Pore in these neurons and whether 3) CyPD inactivation inhibits these effects. In Aim 3, we will determine whether drugs that inactivate CyPD protect axons in EAE and cortical neurons in vitro. Our results will expand our knowledge of how modulation of mitochondrial PT Pore responses influence axonal injury in EAE and MS, facilitating the development of novel neuroprotective therapies for the treatment of MS. PUBLIC HEALTH RELEVANCE: Our research seeks to understand how to prevent damage to nerve fibers in the spinal cord of mice with a multiple sclerosis-like disease. We have found that we can dramatically reduce damage to nerves in this multiple sclerosis-like disease by blocking a protein in mitochondria. The results of this research should lead to new treatment approaches for multiple sclerosis by using drugs to block the mitochondrial protein.